What is a Fossa Ovalis Membrane Aneurysm (Atrial Septal Aneurysm)?
A fossa ovalis membrane aneurysm, also known as an atrial septal aneurysm (ASA), is an excessive bulging or redundant deformity of the interatrial septum, typically at the level of the fossa ovalis, that protrudes into the right atrium, left atrium, or both during the cardiac cycle. 1
Diagnostic Definition
The diagnosis is established when the interatrial septum demonstrates more than 10 mm excursion from the centerline during the cardiac cycle 2. More specifically:
- Excursion threshold: The septum must bulge ≥10-15 mm from the plane of the atrial septum
- Base diameter: At least 15 mm diameter base of the interatrial septum 3
- Membrane characteristics: The fossa ovalis membrane is markedly enlarged (mean 711 mm² vs. 203 mm² in normal hearts) and significantly thinner (0.6 mm vs. 1.9 mm) 4
Anatomical Features
The aneurysm typically presents as:
- Shape: Most commonly dome-shaped 5
- Size: Excursion ranges from 10-35 mm (mean 16 mm), with width 16-40 mm (mean 24 mm) 5
- Direction: Protrudes into the right atrium in 73% of cases, left atrium in 27% 5
- Tissue quality: Characterized by thinning and marked redundancy of the fossa ovalis membrane 4
Prevalence
The frequency varies by detection method:
- Transthoracic echocardiography: 1.9-2.5% in adults 2, 6
- Transesophageal echocardiography: 2-10% in adults 7
- Pediatric population: 0.9-1.7% in children, 4.9% in infants 7
- General population: 2.4% in a large adult outpatient series 3
Clinical Significance and Associated Conditions
An atrial septal aneurysm is often associated with septal fenestrations and may be a marker of increased embolic risk 1. The clinical relevance stems from frequent associations with:
High-Risk Features:
- Patent foramen ovale (PFO): Present in 70% of ASA cases vs. 20% in controls (p=0.0005) 4
- Cryptogenic stroke: Higher prevalence in younger patients with unexplained stroke 2
- Paradoxical embolism: The combination of ASA and PFO creates an anatomic substrate for right-to-left shunting of thrombus 2
Common Concurrent Pathologies:
- Valvular regurgitation: 74% (mitral and aortic) 3, 6
- Supraventricular arrhythmias: 16% 3
- Mitral valve prolapse: 15% 6
- Hypertension: 64% 6
- Stroke: 20% 6
Diagnostic Approach
Transesophageal echocardiography has higher sensitivity for detection compared with transthoracic imaging and is recommended in younger adults with unexplained cerebrovascular events 1.
Detection Method:
- Microbubbles enter the right atrium during saline contrast injection
- If a PFO is present with ASA, bubbles pass into the left atrium within a few cardiac cycles
- Valsalva maneuver increases sensitivity by transiently increasing right atrial pressure 1
Important Caveat:
Estimating PFO size based on contrast amount in the left atrium may be unreliable 1
Pathophysiology
The formation relates primarily to:
- Abnormally elevated intracardiac pressures: In 91% of cases with patent foramen ovale 5
- Congenital redundancy: Excessive tissue at the fossa ovalis 4
- Bidirectional pressure changes: During cardiac cycle and respiration 1
Potential Complications
- Paradoxical embolism: Via stretched patent foramen ovale allowing bidirectional shunting 5
- Thrombus formation: Focal mural thrombus present on aneurysmal surface in some cases 5
- Endocardial fibrosis: Present on the aneurysmal wall in several cases 5
- Arrhythmias: Particularly supraventricular tachycardia 3, 8
- Structural interference: Can narrow the inferior vena caval orifice in rare cases 5
Clinical Context for PFO Closure Planning
Transesophageal echocardiography is recommended for detailed visualization of atrial septal anatomy when PFO closure is planned 1. The presence of ASA may pose technical challenges during percutaneous closure procedures, though direct-suture techniques can effectively close the PFO and reduce aneurysmal redundancy 9.